The invention disclosed herein pertains generally to an apparatus and method for cooling the hot gas casings of combustion chambers and more particularly, the hot gas casings of the combustion chambers of gas turbine power plants.
A gas turbine power plant typically includes a gas turbine, an air compressor, and a combustion chamber. The combustion chamber usually includes a combustion space enclosed by a hot gas casing. The gas turbine, the compressor, and the combustion chamber are usually enclosed in a housing. In operation, the gas turbine is supplied with hot gases flowing from the combustion chamber. In the case of prior art gas turbine power plants compressed air supplied by the compressor flows into the combustion chamber through a space located intermediate the hot gas casing and the power plant housing. Because this compressed air has a lower temperature than the surface of the hot gas casing, the air absorbs heat during its transit through the intermediate space. In this fashion, the hot gas casings of prior art gas turbine power plants are cooled by the compressed air supplied by an air compressor. This is disclosed, for example, in Swiss Pat. No. 284 190.
The method and apparatus used to cool the hot gas casings of prior art gas turbine power plants leads to a nonuniform flow of cooling compressed air over these casings. This nonuniform flow produces quite nonuniform casing wall temperatures which leads to thermal wall stresses, particularly when the turbine inlet temperatures are relatively high. Such thermal stresses can lead to cracks in hot gas casings. These thermal stresses and cracks are particularly evident in the hot gas casings of gas turbine power plants having one or more combustion chambers which are external to the power plant housing.
Accordingly, it is an object of the present invention to cool the hot gas casings of the combustion chambers of gas turbine power plants more uniformly in order to avoid the thermal stresses and cracks produced by nonuniform cooling.
A further object of the present invention is to be able to regulate the cooling of the hot gas casings according to the requirements of the situation.
Apparatus for uniformly cooling the hot gas casing of the combustion chamber of a gas turbine power plant, according to the present invention, includes an outer shell which encircles the hot gas casing to form a cooling air channel between the casing and the shell. This outer shell includes air inlet openings to permit cooling air from an air compressor to enter the cooling air channel. The cooling air channel terminates at a combustion air inlet of the hot gas casing.
Compressed air from the air compressor impinges like a jet on the hot gas casing to produce intense cooling. By appropriately adjusting the air inlet openings in the outer shell, as well as the distance of the outer shell from the hot gas casing, the cooling effect can be regulated according to the requirements of the situation. The cooling air is ultimately delivered to a combustion space of the combustion chamber either by being mixed with a primary air source flowing into the combustion space through a primary combustion air inlet, or by flowing into the combustion space through secondary air inlet nozzles.
A preferred embodiment of the present invention employs an optimum number of cooling air inlet openings in the outer shell, i.e., a number determined by the actual turbine inlet temperature prevailing on the inside surface of the hot gas casing.
A further preferred embodiment of the present invention includes the use of air ducts in the cooling air inlet openings. The use of such air ducts results in a uniform distribution of cooling air over the surface of the hot gas casing. The use of such ducts also reduces the possibility that the cooling air stream will be deflected by air flowing off the surface of the hot gas casing, and provides for a physical separation of the cooling air supply from the flow off.
Yet another preferred embodiment of the present invention includes the use of baffles in the cooling air channel for conducting the cool air between the outer shell and the hot gas casing. The use of baffles produces not only local baffle cooling but also film cooling because the cooling air is forced into intimate contact with the outside surface of the hot gas casing as it flows from baffle to baffle.
Yet a further preferred embodiment of the present invention includes the use of secondary air inlet nozzles arranged over the surface of the hot gas casing in order to pass cooling air directly from the cooling air channel into the combustion space. The relatively high pressure of the cooling air injected into the combustion space through the secondary air nozzles permits an increase in the pressure drop across the combustion chamber. In addition the use of the secondary nozzles imparts a relatively high velocity to the cooling air flowing through these nozzles and consequently produces a better mixing of the cooling air with the air entering the combustion space through the primary combustion air inlet.
Yet still another preferred embodiment of the present invention includes a flow restrictor placed between the outer shell and the power plant housing in the path of the air flowing directly from the air compressor to the primary combustion air inlet. In use, such a restrictor produces an increase in the velocity of that portion of the air stream not used for cooling. In addition, such a restrictor acts as an ejector for the cooling air in the cooling air channel, sucking cooling air through the space between the hot gas casing and the outer shell with relatively low flow losses.
Yet still a further preferred embodiment of the present invention includes openings in the hot gas casing through which cooling air may enter so as to produce additional film cooling on the inside walls of the hot gas casing.